1. Field of the Invention
This invention relates generally to a stabilization mechanism for use in oxidation/ reduction catalyst systems. It particularly relates to a ruthenium stabilization mechanism that enables the use of inexpensive metallic species within catalyst systems targeted for the elimination of toxic emissions such as carbon monoxide, hydrocarbons and other volatile organics, and specifically nitrogen oxide species.
2. Description of the Related Art
Internal combustion engines are the primary source of air-born pollutants in the United States and many parts of the world. Only since the 1970's have regulatory organizations led by the Environmental Protection Agency (EPA) and sanctioned by the Clean Air Act of 1973 begun initiating mandates for pollution control on ground vehicles. Performance requirements focus on the destruction of the three primary pollutants present in the exhausted emission stream: 1) hydrocarbons (i.e., uncombusted fuel, HC), 2) carbon monoxide (CO), and 3) nitrogen oxides (NOx).
EPA and California Air Resource Bureau (CARB) certification of automotive catalytic converter systems requires a catalyst technology to meet minimum performance requirements for a period commensurate with its final application. Gasoline-powered automobile aftermarket catalysts, for example, must currently eliminate 70% of emitted CO, 70% HC, and 60% NOx for a period of 25,000 miles operation (OE market>100,000 miles). Product durability is evaluated through approved rapid aging tests (RAT) that involve subjecting the catalyst to the emissions stream of a full-scale automobile engine operating under elevated inlet temperatures (e.g., 850° C.) and varying fuel-to-air ratios ranging from rich to lean relative to stoichiometric (i.e., ideal) operating conditions. Catalytic converter performance is characterized by the efficiency of the conversion process and product durability in maintaining the mandated pollutant destruction levels.
Catalytic converters are complex, highly proprietary systems, all exploiting precious metal (PM) species as the primary catalytic elements. Catalyst systems for automotive converter applications, typically called three-way catalysts, consist of at least four primary components: 1) substrate, 2) inactive support material, 3) active oxygen storage material, and 4) precious metal (PM) species such as the primary oxidation—platinum (Pt) and palladium (Pd) and reduction—rhodium (Rh) and iridium (Ir) metals. Substrates are typically honeycomb geometries of a mixed metal oxide composite (e.g., cordierite) or in a few cases, metal alloys (e.g., FeCrAl). Standard automobile converters typically use a single cordierite brick (e.g., 400 cells/square inch) of volume commensurate with engine size.
The inactive wash coat (eg., aluminum (oxide, alumina)), oxygen storage material (e.g., cerium oxide, ceria), and some cases other metal species (e.g., iron, cobalt, nickel) used to facilitate electron transfer in the insulating support material are applied to the substrate from a particles-in-solvent slurry wash coat process. Following drying and calcining treatments at temperatures typically not exceeding 550° C. for six hours, precious metals are applied to the oxide coating quantitatively from salt solutions. Product assembly then follows an additional thermal treatment.
The key problem is that PMs are scarce commodities found naturally in quantity only in isolated regions of the Asian and African continents. Moreover, the dynamic market of individual PM species has driven, and will continue to dictate, the direction of catalyst technology developments. As a result, it is economically prudent and environmentally responsible to pursue the development of next generation catalyst systems that exploit more efficient active support materials and more readily available (i.e., less costly) metallic species for pollutant destruction.
NASA Langley Research Center (LaRC) has developed a tin oxide/ceria-based three-way catalyst technology described in U.S. patent application Ser. No. 10/056,845 filed Jan. 22, 2002, and hereby incorporated herein as if set forth in its entirety. The present invention uses ruthenium metal for NOx remediation, an attractive alternative to Rh from an availability, and hence cost perspective, often trading at prices less than Rh by an order of magnitude. Ruthenium reduces NOx as well as supports the oxidation of CO and HC; however, its use was originally abandoned by the industry following research in the 1970's that showed its predilection to form volatile and toxic ruthenium oxide species that would be emitted into the environment. Therefore, exploiting ruthenium in three-way catalysts for NOx destruction required the stabilization of ruthenium under the high temperature, oxidizing conditions of the automotive exhaust system.